1. Field of the Invention
The present invention relates to an exposure system which transfers a pattern formed on a mask or a reticle onto a substrate such as a wafer, and to a method for manufacturing devices such as a semiconductor element, a liquid crystal displaying element, an image-capturing element, a thin film magnetic head, and other devices by using the exposure system.
The present application is based on Patent Application No. 2002-122567, filed in Japan, the content of which is incorporated herein by reference.
2. Description of Related Art
An exposure apparatus which transfers a pattern which is formed on a mask or a reticle (hereinafter generally referred as a mask) onto a wafer or a glass plate, etc., on which a photosensitive agent such as photoresist is applied is used in a photolithography step which is a step for manufacturing a semiconductor element, a liquid crystal displaying element, an image-capturing element, a thin film magnetic head, and other devices.
For an exposure apparatus which is used commonly in recent years, there is an exposure apparatus which employs a step and repeat method such as a reduction projection exposure apparatus (a so called stepper) and an exposure apparatus which employs a step and scan method. An exposure system employing the stepped and repeat method is an exposure apparatus which moves a wafer in a step manner and repeats a movement which transfers a pattern which is formed on a reticle onto each shot area at once. Also, an exposure system employing the step and scan method is an exposure apparatus which repeats a movement which transfers a pattern which is formed by a reticle to a shot area successively for every shot area while moving the reticle and the wafer synchronously.
In recent years, in particular, semiconductors have become highly integrated; therefore, improvement in resolution by realizing a shorter wavelength for an exposure light and by realizing a high NA (Numerical Aperture) of an optical projection system, and an improvement in an overlapping accuracy by an accurate and strict control of a baseline amount are desired. Here, a baseline amount is a distance between a reference point (for example, a center of projection) for a pattern image of the reticle which is projected on the wafer and a reference point (for example, a center of measured perspective) of an alignment sensor according to an off-axis method.
Also, an improvement in productivity is required for a production of a device in recent years. An acceleration for a reticle stage and a wafer stage increases so as to improve a through-put (number of wafers which are processed per unit time); thus, heat generated in a motor which drives these stages increases. The motor which drives the stage is disposed inside the exposure apparatus; therefore, the temperature in the exposure apparatus fluctuates to a great extent every time the exposure operation is repeated. In particular, both the wafer stage and the reticle stage are driven by a motor during an exposure operation in the exposure apparatus according to step and scan method; therefore, the temperature fluctuates to a greater extent. Here, the varies in the exposure apparatus not only due to motor which are disposed in the wafer stage and the reticle stage but also due to a driving system (for example, a lens driving system of the optical projection system and a reticle blind driving system) in the exposure apparatus are driven.
If the temperature in the exposure apparatus varies, the resolution may be reduced by variation (for example, fluctuation in the best focusing position and a variance in aberration) of the optical characteristics of the optical projection system. Also, a baseline amount may fluctuate due to thermal expansion and thermal deformation of the alignment sensor and the stages; thus, an accuracy for overlapping the projected pattern and the shot area may be reduced. In order to avoid these disadvantages, an exposing system is formed by an exposure apparatus and a temperature controlling device which maintains the temperature in the exposure apparatus. In recent years, it is common for an exposure system to be provided with a large-scale temperature controlling device which is according to a liquid cooling method in which cooling capacity is high. In such a case of preventing an increase of installation area (footprint) of the exposure system in a clean room, it is common that at least a part of the temperature controlling device is disposed under (also called “under a floor surface” or “under an installation surface” in this specification) a floor surface (an installation surface) on which the exposure apparatus is disposed.
In addition, in a case in which the temperature controlling device is disposed in the above-explained manner, an object (reticle stage, wafer stage, etc.) of which the temperature is supposed to be controlled is disposed higher than a floor surface on which the exposure apparatus is disposed, and a tank which collects the cooling medium which is circulated in the object of which the temperature is supposed to be controlled, a pump which circulates the cooling medium, and the temperature adjusting device which sets the temperature of the cooling medium are disposed under the floor surface. Therefore, the cooling medium is compressed so as to be transported by the pump to the height position of the object of which temperature is supposed to be controlled. Consequently, the cooling medium is collected in the tank after passing the object of which temperature is supposed to be controlled and falling therefrom due to a vertical positioning difference; thus, the cooling medium is compressed to be circulated in a path by the pump again.
In the controlling device which has the above-explained circulation path, the cooling medium descends after passing the object of which the temperature is supposed to be controlled; therefore, there may be a case in which a negative pressure is generated at a position of the object of which the temperature is supposed to be controlled. For example, a larte negative pressure is generated when a circulation of the cooling medium is stopped in the circulation path. When this negative pressure is generated, a case may occur in which some kinds of deformation may occur in the cooling pipe (which forms a part of the circulation path) which is disposed in the reticle stage and the wafer stage as an object of which the temperature is supposed to be controlled; thus, there may be a concern that an operation performance of the stage will be undesirably affected. As an example, in a case in which a cooling pipe is disposed in a linear motor (moving-magnet type) which drives the stage, if the cooling pipe is deformed by the negative pressure, a gap between the fixed element (coil) and the cooling pipe is unstable. As a result, the fluid amount of the cooling medium which flows in the gap is unstable; thus, temperature stability is unstable in the motor. Therefore, there are concerns that a stability of a thrust force of the motor may be unstable, or there may be a temperature distribution on a surface of the cooling pipe; thus, there is undesirable influence on accuracy for overlapping the baselines.
Also, in a case in which the temperature controlling device is disposed in the above-explained manner, it is necessary to eliminate air which remains in the circulation path so as to fill the circulation path with the cooling medium. If the pump is a large scale pump which has a high compressing performance, it is possible to raise the cooling medium to the highest position of the circulation path even if the position which supplies the cooling medium to the circulation path and the position for eliminating the air are lower than the highest position of the circulation path. Therefore, it is possible to eliminate the air from the circulation path so as to fill the circulation path with the cooling medium there.
On the other hand, in a case in which a small pump must be used due to a limitation such as that due to an installation area, a method is used which supplies the cooling medium from a vicinity of the highest position of the circulation path into the circulation path and eliminating the air from the vicinity of the highest position of the circulation path so as to fill the circulation path with the liquid (the cooling medium) by gravity and only so as to use the pump as a circulating capacity. However, air cannot necessarily be removed from the highest point of the circulation path if this method is used; therefore, there may be a case in which it is not possible to eliminate air in the circulation path completely. Moreover, there is a case in which air remains there for unknown reasons. If air remains in the circulation path, a back pressure is generated in the circulation path; therefore, if a small pump is used which does not have a sufficient compressing capacity so as to resist the force of air which ascends upwardly, there is a concern that a cooling capacity for the stage may be reduced because it is no possible to flow a desirable fluid amount in the circulation path as a result. In addition, there is a concern that an operability performance of the stage may be reduced. Also, quite often, a cooling pipe which does not have a high pressure resistance from a space point of view is used for a linear motor, etc.; thus, there is a problem in that it is not possible to increase the fluid amount of the liquid when the back pressure increases in the circulation path.